1. Field
One or more embodiments relate to an encryption apparatus, a method for encryption, a computer-readable recording medium, more particularly, to an encryption apparatus capable of performing homomorphic encryption on a ciphertext which is encrypted by using a public key method, a method for encryption, a computer-readable recording medium.
2. Description of the Related Art
Since the quantity of data processed by individuals and corporations increases, data needs to be efficiently managed. Accordingly, recently the number of cases goes up where information is stored by utilizing external database service.
Meanwhile, in case where sensitive information such as personal information, etc. is stored in an external device, a problem such as data spill, etc. could occur, which requires data to be encrypted to be stored.
When data is encrypted to be stored as described above, security on the stored data is guaranteed. A database provides an application environment in which data are searched and utilized, in addition to data storing. However, encrypted data prohibits the database server from obtaining any information, and thus such application service is prohibited for encrypted data, from the beginning.
The most widely known scheme among the homomorphic encryption is fully homomorphic encryption (FHE). According to FHE, only a single bit can have a plaintext and supports addition and multiplication between ciphertexts. The FHE is a scheme in which somewhat homomorphic encryption which supports a limited number of addition and multiplication is designed first and then a new ciphertext whose noise has been reduced is reduced by using squashing and bootstrapping.
However, because a ciphertext of the conventional FHE is too large in size, there has been a disadvantage that the ciphertext cannot be used in a mobile terminal. For instance, the smallest size of a ciphertext among the FHE algorithm which has been proposed is 460 kB, which is Ring LWE-based encryption. The Ring LWE-based encryption is 100 times as large as the size of the encryption according to the existing encryption system (ElGamal, Paillier, RSA, AEX).
Accordingly, recently, a hybrid encryption scheme which uses a method of combining the existing encryption system with FHE which supports operations between encryptions has been proposed.
The hybrid encryption scheme transmits/stores common ciphertext having relatively very small size, and thus the hybrid encryption scheme is efficient in terms of space (bandwidth, memory). Until now, only hybrid encryption scheme which combines a symmetric-key cryptosystem such as the Advanced Encryption Standard (AES) with FHE has been suggested. However, the AES can be applied only to personal DB, which results in disadvantage in terms of applicability.